(1) Field of the Invention
The present invention relates to a light emitting element driving apparatus, and more particularly one for performing automatic light power control (APC control; Automatic Power Constant control) for a light output from a light emitting element.
(2) Description of the Related Art
In recent years, we have seen active efforts made to develop subscriber exclusive optical communication devices in the field of optical communications. In an optical communication device used in an optical subscriber system, a semiconductor circuit based on a CMOS type field effect transistor has been put to frequent use in order to satisfy requests for reductions in costs, power consumption, and so on.
Not only in the foregoing optical communication device of the optical subscriber system but also in the optical transmission device of a trunk system, a function unit for transmitting light signals via optical fibers transmits signals containing data information, for instance burst signals, by driving a light emitting element such as a laser diode (LD).
In such a situation, a light emitting element driving apparatus for driving the light emitting element such as an LD must have a function for dealing with burst signals and, more importantly, a function for accurately maintaining light power constant even if there is a long time interval between burst signals. These functions must be provided also for securing reliability of the light emitting element itself.
In a driving apparatus for driving a magneto-optic disk (MO) or a laser printer, an LD or the like is used as a light emitting element for emitting laser lights. For the magneto-optic disk driving apparatus or the laser printer, functional improvements can be expected by providing a function for accurately maintaining light power constant even if a long time interval occurs between burst signals.
FIG. 22 is a block diagram showing a light emitting element driving apparatus where a conventional automatic light power control circuit is applied. A light emitting element 100 and a light emitting element driving apparatus 110 shown in FIG. 22 can be applied to an optical communication device for transmitting/receiving light signals via not-shown optical fibers.
The light emitting element 100 converts electric signals into light signals, supplies output signals to a not-shown optical transmission line and outputs monitoring lights. The light emitting element driving apparatus 110 performs control so as to maintain constant light power of a light signal outputted from the light emitting element 100. The light emitting element driving apparatus 110 includes a data receiving unit 101, an LD driving unit 102, a reference voltage generation unit 103, a light receiving element 104, a monitoring voltage conversion unit 105, a difference voltage generation unit 106 and a control signal generation unit 107.
The data receiving unit 101 receives data and a clock to produce a signal for driving the light emitting element 100. The LD driving unit 102 receives an output from the data receiving unit 101. The LD driving unit 102 is controlled by a control signal produced by the control signal generation unit 106 for automatic light power control so as to drive the light emitting element (LD) 100. The data receiving unit 101 and the LD driving unit 102 constitute a main signal unit 108 together.
The reference voltage generation unit 103 generates a reference voltage for automatic light power control from a reference signal as an output signal from the data receiving unit 101. The light receiving element (PD; Photo Diode) 104 converts a monitoring light outputted from the light emitting element 100 into an electric signal again. The monitoring voltage conversion unit 105 voltage-converts a monitoring signal as an output current from the light receiving element 104 so as to produce a monitoring voltage for automatic light power control.
The difference voltage generation unit 106 produces a difference in output voltages between the reference voltage generation unit 103 and the monitoring voltage conversion unit 105. The control signal generation unit 107 produces an LD driving control signal for automatic light power control according to the output of the difference voltage generation unit 106. The reference voltage generation unit 103, the monitoring voltage conversion unit 105, the difference voltage generation unit 106 and the control signal generation unit 107 constitute an automatic light power control unit (APC unit) 109 together.
Detailed configuration of each of the LD driving unit 102, the difference voltage generation unit 106 and the control signal generation unit 107 is shown in FIG. 23.
Specifically, the difference voltage generation unit 106 includes a differential amplifier. The control signal generation unit 107 includes a field effect transistor (T1) 107a, a capacitor (C1) 107b, a field effect transistor (T2) 107c and a resistor (R2) 107d.
The field effect transistor 107a supplies a current according to the output of the difference voltage generation unit 106. The capacitor 107b is connected to the field effect transistor 107a via a connector 107e. The capacitor 107b charges a current supplied from the field effect transistor 107a. A terminal voltage of the capacitor 107b is outputted as a control signal for the LD driving unit 102.
The field effect transistor 107c is placed in a conductive condition when a transmitting signal is ON. The resistor 107d causes the capacitor 107b to discharge excessive electric charges when a transmitting signal is ON.
In other words, the field effect transistor 107a and the capacitor 107b produce a control signal for the LD driving unit 102 according to an output from the difference voltage generation unit 106.
The LD driving unit 102 includes three field effect transistors (T11 to T13) 102a to 102c and a resistor (RL) 102d. An APC control signal from the control signal generation unit 107 is received by the transistor 102c. Data transmitted from the data receiving unit 101 is received by the transistors 102a and 102b. Then, a driving current signal having been subjected to automatic light power control is supplied to the light emitting element 100.
With the foregoing configuration, the light emitting element driving apparatus 110 shown in FIG. 22 controls a driving current of the light emitting element 100 based on a difference voltage between a reference voltage produced from a signal outputted from the data receiving unit 101 and a monitoring voltage produced from a monitoring signal outputted from the light receiving element 104, controls an output light of the light emitting element 100 to a constant level and outputs the output light to the optical transmission line.
In the control signal generation unit 107, if a transmitting signal (burst signal) is in an ON condition (transmission condition) [e.g., see points of time (t2) to (t4) of FIGS. 24(b) and 24(c)] after power input [see a point of time (t1) of FIG. 24(a)], the transistor 107c is switched ON. Accordingly, a control signal V.sub.PCNT for the LD driving unit 102 is controlled according to the light output control of a loop gain including the resistor 107d [see points of time (t2) to (t3) of FIG. 24(d)].
On the other hand, if the transmitting signal is in an OFF condition (non-transmission condition) [e.g., see points of time (t4) to (t5) of FIG. 24(b)], the transistor 107c is switched OFF. Accordingly, a control signal V.sub.PCNT produced during transmission [see points of time (t2) to (t4) of FIG. 24(b)] is held until a next burst transmission section [see a point of time (t5) and after of FIG. 24(b)] is reached.
FIG. 25 is a block diagram showing a light emitting element driving apparatus 110A where an APC loop is composed of a digital circuit. In the light emitting element driving apparatus 110A shown in FIG. 25, a light emitted from the light emitting element 111 in a rear direction is made incident on the light receiving element 112. The light receiving element 112 outputs a current proportional to its light intensity. This current is converted into a voltage by the amplifier 113 and then compared with a reference voltage Vref from a not-shown voltage source by a comparator 114.
An output voltage of the comparator 114 is set to a high level or a low level depending on a size relationship between both input voltages of the comparator 114. For example, if a voltage signal from the amplifier 113 is larger than the reference voltage Vref, the level of an output signal of the comparator 114 can be set low. If a voltage signal from the amplifier 113 is smaller than the reference voltage Vref, the level of an output signal of the comparator 114 can be set to an optical level.
An edge detector 115 detects an edge of a transmission switching signal as a transmission or non-transmission timing for a transmitting data (burst signal). In the case of a non-transmission timing for transmitting data, the edge detector 115 outputs a signal for placing a rear-stage up-and-down counter 116 in an enable condition. In the case of a transmission timing for transmitting data, the edge detector 115 outputs a signal for releasing the enable condition of the up-and-down counter 116.
During data transmission, the up-and-down counter 116 counts a comparing result from the comparator 114 in synchronization with a clock signal from an oscillator 117. For example, if a voltage signal from the amplifier 113 is larger than the reference voltage Vref, the up-and-down counter 116 counts down a count value. On the other hand, if a voltage signal from the amplifier 113 is smaller than the reference voltage Vref, the up-and-down counter 116 counts up a count value.
For example, if light intensity from the light emitting element 111 is weaker than a reference value, an output of the comparator 114 becomes low in level and the up-and-down counter 116 operates as an up-counter. When the edge detector 115 releases an enable signal supplied to the up-and-down counter 116 based on a timing signal T1 such as a transmission switching signal, the up-and-down counter 116 gradually increases its measuring values by means of a clock signal from the oscillator 117.
A counted output from the up-and-down counter 116 is converted into an analog amount by a D/A converter 118 and then outputted to a semiconductor laser driving unit 119. The semiconductor laser driving unit 119 changes a driving current level for converting transmitting data of an electric signal into an optical signal according to an output of the D/A converter 118.
Therefore, during transmission of transmitting data, the up-and-down counter 116 functions as an up-counter until a voltage signal from the amplifier 113 exceeds the reference voltage Vref in size. As count values of the up-and-down counter 116 are gradually increased, light intensity from the light emitting element 111 is gradually increased and an output from the amplifier 113 is also increased.
When an output of the amplifier 113 exceeds the reference value Vref and a comparing result signal from the comparator 114 is reversed from a low level to a high level, the edge detector 115 detects a rising edge of an output from the comparator 114 and then outputs an enable signal to the up-and-down counter 116. Accordingly, the up-and-down counter 116 is placed in an enable condition to hold its count value and a driving current to be supplied to the light emitting element 111 is also held therein.
However, in the foregoing configuration for holding light power by the analog circuit which uses a MOS element or the like shown in FIG. 22, it is difficult to prevent a reduction in performance caused by a drain leak current or the like of the MOS element.
In other words, in the light emitting element driving apparatus 110 shown in FIG. 22, during non-transmission of transmitting data, regardless of the necessity of holding a voltage, the capacitor 107c is further charged because of the occurrence of a leaked current [see points of time (t4) to (t5) of FIG. 24(d)] from the transistor 107a.
In such a case, a current more than necessary is discharged at the head of a next burst transmitting signal. Consequently, the light emitting element 100 may be driven by an excessive current to cause an output optical level to exceed a reference level [see a point of time (t5) of FIG. 24(f)].
Charging of the capacitor 107c by such a leaked current is more conspicuous as an interval with the next burst transmitting signal is longer (longer holding time). Consequently, fluctuation may occur in power at the head of the next burst transmitting signal to make it impossible to meet pulse mask standard.
If a capacitance of the capacitor 107c is increased in order to reduce the influence of power fluctuation caused by the leaked current, a charging time for the capacitor 107b may be slowed down. Thus, an initial rising time of a control signal V.sub.PCNT by the control signal generation unit 107 at the head of a first burst may also be slowed down.
A current of the transistor 107a may be increased to make faster an initial rising time. In this case, however, the transistor size itself of the field effect transistor 107a must be increased. Consequently, a leaked current of the transistor 107a is increased more, which makes it impossible to prevent power fluctuation at the head of the next burst transmitting signal.
As apparent from the foregoing, in the light emitting driving apparatus 110 shown in FIG. 22, the APC loop is composed of the analog circuit and thus if an error occurs in a control signal outputted from the control signal generation unit 107, light power fluctuation may directly occur.
On the other hand, by composing an APC loop of a digital circuit as in the case of the light emitting element driving apparatus 110A shown in 25, it is possible to prevent light output power fluctuation caused by the foregoing configuration where the APC loop is composed of the analog circuit.
In other words, the light emitting driving apparatus 110A shown in FIG. 25 prevents, by performing control to hold a driving current between burst signals based on a digital signal, power fluctuation like that which occurs at the head of a burst transmitting signal after long-time holding by a leaked current in the light emitting driving apparatus 110 shown in FIG. 22.
However, there is a problem inherent in the light emitting element driving apparatus 110A shown in FIG. 25. Specifically, at an initial rising time of a first burst signal after power input [see a point of time (t11) of FIG. 26(a)], the up-and-down counter 116 performs up-counting only by 1 bit each in synchronization with a clock signal and an output voltage of the D/A converter 118, i.e., an LD driving control signal, is raised only by an amount equal to resolution of the D/A converter 118, the resolution being 1 LSB. Consequently, unnecessary time is required until an optional stable light output level is reached [see points of time (t12) to (t13) of FIG. 26(d)].
Resolution of the D/A converter 118 may be increased to achieve a high speed for an APC initial rising time. In this case, however, the occurrence of errors during an APC normal operation is increased. Thus, a loop compression residual may occur, which leads to power fluctuation.
In the digital APC using the up-and-down counter 116, because of the unstable condition of an output of the comparator 114 during an APC loop unstable operation (after convergence), an unstable operation may occur.
Specifically, when light output power nearly reaches its objective power [see points of time (t13) to (t14) of FIG. 26(e)], near coincidence can also be reached between the reference voltage Vref and an output of the amplifier 113. However, because of the occurrence of offsets or noises in the reference voltage Vref [see points of time (t13) to (t14) of FIG. 27(a)] or a conversion error in the D/A converter 118, an output voltage of the comparator 114 becomes a high level or a low level in an unstable condition. Consequently, variance may occur. [see points of time (t13) to (t14) of FIG. 27(b)].
As described above, if variance occurs in outputs of the comparator 114 after light output power nearly reaches its objective power and the APC loop is completed, the up-and-down counter 116 performs up-counting or down-counting optionally and, consequently, an operation condition becomes unstable.
A control signal as an output signal of the D/A converter 118 for the semiconductor laser driving unit 119 is used for an operation performed for each resolution (1 LSB). Thus, when a count value of the up-and-down counter 116 becomes unstable, fluctuation also occurs in an output of the D/A converter 118 in the vicinity of an objective voltage [see points of time (t13) to (t14) of FIG. 27(c)]. Consequently, an unstable operation such as light power fluctuation may occur [see points of time (t13) to (t14) of FIG. 27(d)].